Within the last decade the use of wireless communications systems, such as cellular telephones, has increased at very rapid rate and today most countries around the world have established wireless systems within their boundaries. These wireless systems vary in mode of operation from country to country (and even region to region with the same country) because of different bandwidth, power and frequency requirements. As an example in digital cellular telephone operation, the United States mostly employs digital cellular systems based on either Code Division Multiple Access (CDMA) or Time Division Multiple Access (TDMA) modes of operation while Western Europe generally employs digital cellular systems based on the Global System for Mobile Communications (GSM) mode.
At present, there exists a need for wireless telecommunications systems that are capable of operating with multiple modes and/or modulations such as cellular telephones which can operate in a variety of systems supporting different modulation formats. As an example, a need exists for a cellular telephone that can operate in TDMA and CDMA modes of operation. However, it is appreciate by those skilled in the art that each mode presents a unique constraint on the power amplifier of the cellular telephone due to the differences in transmitted power level, post power amplifier insertion loss, and modulation-dependent linearity and efficiency requirements of each mode. Such cellular telephones pose a significant challenge to design and build because, for proper operation, the power amplifier must have differing output load-lines for each modulation format.
Proposals for solving this problem in the past included having separate power amplifiers for each modulation format, and then switching between these amplifier chains. This involves duplicated circuitry that took up more space and increased the cost of the systems. To add complexity to this problem, many future cellular telephones will need designs accommodating still more modulation formats (as new formats develop), making the engineering even more challenging.
U.S. Pat. No. 5,060,294 ('294), issued to Schwent et al., discloses a means for altering the load characteristics on a power amplifier to either cause amplifier saturation or prevent amplifier saturation. However, in the future cellular telephones will have the ability to operate in multiple linear modes such as linear TDMA (e.g. employing phase shift keying modulation) and some form of CDMA. The '294 patent does not address cellular telephones that operate between different linear modulation formats.
Further, the '294 patent discloses altering a bias current of an amplifier dependent upon whether the amplifier operates in a saturated or a non-saturated mode. As stated above, the '294 patent does not discuss cellular telephones that operate between different linear modulation formats. Additionally, altering an amplifier bias current provides a first order of amplifier performance improvement. There is a need for more efficiency and/or linearity improvement.
U.S. Pat. No. 5,291,516 issued to Dixon et al. discloses a dual-mode transmitter with an adjustable output filter depending upon the mode of operation. This addresses issues such as signal splatter, radiated emissions, and radiated noise power, but it does not provide the needed performance improvements to amplifier efficiency and/or linearity.
U.S. Pat. No. 5,361,403 issued to Dent discloses an amplifier that switches between a first load impedance when the amplifier is amplifying an AM signal at a first power level and a second load impedance when the amplifier is amplifying an FM signal at a second power level. However, there is a need for cellular telephephones to operate via different linear modulation schemes. For example, these different linear modulation schemes can be different forms of digital modulation including different forms of quaternary phase shift keying (QPSK), binary phase shift keying (BPSK), and/or quadrature amplitude modulation (QAM). These different digital forms of modulation have different peak-to-average ratio characteristics that impact amplifier efficiency in specific ways.
Therefore, there exists a need for an amplifier that operates in multiple modes of operation and significantly reduces the complexity, space, and cost of manufacturing and increases amplifier performance.